As discussed in detail in my U.S. Application Ser. No. 773,616 filed Sept. 9, 1985 and entitled "Method and Apparatus for Placing Cement Plugs in Wells", now U.S. Pat. No. 4,674,573, issued June 23, 1987, cement plugs are placed in wells for various purposes, for example when a well is to be abandoned, or when a well bore is to be "kicked-off", during drilling, along a different directional path. A known volume of cement slurry is forced down a pipe string by a following displacement fluid, with a plug used as an interface to separate the slurry and displacement fluid. After the slurry has been pumped into the annulus in the region surrounding the lower section of the pipe, the pipe is withdrawn, leaving a column slurry at the bottom of the well bore to set up and harden to form a solid plug that bridges and seals off the well bore.
In order that a sufficient and controlled amount of slurry be used to form a cement plug of a specified height, the typical procedure is to place in the pipe a calculated volume of slurry, and then position a fluid displacement plug (sometimes called a "dart") at the top of the slurry column. A displacement fluid under pressure is then employed to drive the plug downward through the pipe with the slurry ahead of it, until the plug enters a "catcher" sub at the lower end of the pipe string. When this occurs, there will be virtually no slurry left inside the pipe string, which will have been wiped clean of slurry by the plug as it passes downward therein.
Skill is required during the cement plugging operation to know precisely when to stop the pumps so that the displacement plug or dart has reached a known location in the pipe string. Various restriction devices have been used which cause an observable pressure surge to occur at the surface as the plug passes therethrough. One device is constituted by a pipe nipple having a reduced diameter throat section. A pressure increase is noted at the surface when the plug enters the top of the throat section, and when the plug passes out of the bottom of the throat the pressure is reduced. This device can be somewhat expensive to manufacture, and is lengthy. Another known restriction device includes an elastomeric sleeve bonded within an internal recess formed in a tubing. Where water-based drilling muds are used as a displacement fluid, a reliable surface indication is given of the passage of a displacement plug. However, when an oil-based mud is used, the resulting slickness of the sleeve can permit the plug to pass through without any significant increase in pump pressure being observed. The same problem exists when a typical dart-type plug made of an elastomeric material is pumped through a metallic restriction formed by the inner bore of a sub, or by a separate metal sleeve fixed within the bore.
An object of the present invention is to provide a new and improved restriction sub of the type described that enables a distinct surface indication to be given, regardless of the characteristic properties of the displacement fluid being used, or of any other fluids present in the well bore.